Direct thermal printers equipped with a line thermal head have been well known. Into such a direct thermal printer, heat-sensitive sheets in the form of cut sheets are stacked up and loaded. Some of such printers are configured to accommodate a sheet package in which numbers of heat-sensitive sheets are stacked up and wrapped by a package member.
In the printer, the sheets are fed into the thermal head one by one and each line orthogonal to the sheet feed direction is heated by the thermal head, by which arbitrary letters, images, etc. are printed on the sheets. As the heat-sensitive sheets, various types of sheets such as heat-coloring sheets and heat-perforated sheets can be used.
The heat-coloring sheet includes a base layer such as paper, a heat-sensitive record layer formed by applying a heat-coloring material on the base layer, and an overcoat layer formed by applying ultraviolet-curing resin, etc. on the heat-sensitive record layer and irradiating the resin with ultraviolet rays. When such a heat-coloring sheet is heated by the thermal head from the side of the overcoat layer (print surface), the heat-sensitive record layer in the heated part changes color. As a type of heat-coloring sheet, there exists a multicolor heat-sensitive sheet having a plurality of coloring layers. When the thermal head heats such a multicolor heat-sensitive sheet from the side of the coloring layers (print surface) while adjusting the amount of thermal energy, the heat-sensitive record layer in the heated part changes color accordingly.
The heat-perforated sheet includes a thermoplastic film, a porous base, and an adhesive layer for bonding them together. When such a heat-perforated sheet is heated by the thermal head of the printer from the side of the thermoplastic film (print surface), the thermoplastic film is perforated to change color of the sheet in a desired pattern (character string, etc.).